Digital electronics including digital and radio frequency (RF) communication circuits are becoming increasingly more ubiquitous in modern life. Many digital and RF communication circuits rely on high-quality frequency sources, or oscillators. For example, oscillators can provide a clock for digital circuits and act as a local oscillator in RF communication systems.
A high purity oscillator typically includes a high quality factor (Q) sharp filter. A higher Q can lead to lower noise in the oscillator and/or power consumption. Filters are also used as standalone components in RF systems to select different frequency bands and channels.
Filters based on mechanical resonance usually have higher quality factors than those of electrical filters. For example, piezoelectric quartz crystals, which can have quality factors greater than 100000 and reasonable frequency stability, are a good example of mechanical filters that dominated oscillators for decades. However, miniaturization and integration of quartz crystals in standard IC technology, specifically complementary metal-oxide-semiconductor (CMOS), can be challenging. Furthermore, quartz crystals are typically bulky, thereby consuming extra space on a system board along with dedicated pins for the integrated circuit. The oscillation frequencies of quartz crystals are typically in the MHz range (e.g., f0<300 MHz). As a result, a phase-locked-loop (PLL) is usually employed in order to generate GHz frequency signals, thereby complicating the system and increasing power consumption.
On-chip electrical L-C tank circuits can be an alternative to mechanical filters for scaling to GHz frequencies. However, the resulting quality factor is usually very low (e.g., Q<30) and it can be challenging to control the resonance frequency due to substantial CMOS process variations. They also tend to consume large on-chip prime die area (e.g., about 100×100 μm2), thereby increasing the size and cost of the CMOS die and accordingly the cost of the overall system.
Micro-electro-mechanical systems (MEMS) resonators are potential candidates to satisfy today's technological demands. MEMS resonators can span a wide frequency range from about 100 kHz to over 10 GHz. They can achieve quality factors exceeding 104, within a footprint that is about 1000 times smaller than that of on-chip L-C tank circuits. Finally, they have the potential for monolithic integration with commercial CMOS IC technologies.
To fully explore the capabilities of MEMS resonators, there remain a few challenges. First, fabrication of most traditional MEMS resonators includes a release step. More specifically, sacrificial layers are included during the micro-machining of MEMS resonators and then etched away to create the freely suspended and vibrating structure for resonators and inertial sensors. The release operation can dramatically affect the process yield.
Second, as mechanical devices, MEMS resonators typically have free surfaces that vibrate and move. As a result, MEMS resonators can be sensitive to ambient pressure and humidity adsorption, as well as particle deposition. These factors can directly affect the quality factor and the resonance frequency of the resonators. In addition, some MEMS resonators include air gaps for electrostatic actuation. These MEMS devices are then usually hermetically sealed and protected from the environment to avoid degradation of the device performance. The sealing can be specialized and costly, thereby increasing the overall system size, complexity, and cost.
Third, true monolithic integration of MEMS resonators with CMOS circuits have been a challenge so far. Integration techniques usually include extensive post-processing or complicated protection at different stages of the fabrication process. Process thermal budget and yield optimization can be major issues for these processes.